1. Field of the Invention
The present invention relates to an improved automatic document conveying device for use with a recording device, an image read out device, or the like in an electrophotographic copying machine.
2. Description of the Prior Art
Automatic document conveying devices are usually mounted on the document table glass (platen glass) of an image exposure section of a copying machine for use. In an optical movable-type copying machine on which this automatic document conveying device is mounted, documents in sheet form (hereinafter, simply document), of which one sheet at a time is removed from a document loading table (stacker section) on which the documents are loaded by means of an automatic document conveying device, are fed onto a document table glass by a conveyor belt. Here, after being exposed by a exposure lamp in the body of the copying machine, each document is discharged onto a document discharge table.
In a document change operation on the document table glass for this type of automatic document conveying device, conventionally the discharge of the document onto the document table is detected by a paper discharge sensor, after which the next document from the stack section is fed out, conveyed onto the document table glass, and halted at a specified position. Because of this considerable time is required to change documents, and accordingly, a long time is spent handling copies, so copy productivity drops.
During a document change operation in which a document is automatically fed to and removed from the document table using an automatic document feed device, if the document change is carried out within the time allowed for the scanner optical system to return, the copy productivity is 100% and there is no time wasted. However, the smaller the document becomes, the wider the gap from document to document, so there is a loss in conveying time.
For this reason, the document conveying speed from the document double feed prevention position in the stack section to the document stop position on the glass surface of the document table must be high.
However, running the document conveyor at high speed causes a large number of problems such as the necessity for a large motor, increased consumption of power, production of noise, increased wear on the conveyor belt, increased paper loss through damage, deterioration of document separation performance, and the like.
To eliminate these problems, conventional technology disclosed in Japanese Laid Open Patent Application 1-236136 provides a document conveying device with a short document change time.
In this document conveying device, a special feature is that, when the first document is conveyed to a specified position and halted at that position, the second document is conveyed to and stopped at a position which is not upstream on the plate, separated from the specified position. Next, when the first document after the complition of the exposure process is at a position from which it can be conveyed by the conveying means to a downstream position on the platen separated from the specified position, at the same time, the second document is conveyed to a specified position and halted at that position. This procedure is repeated with the third and subsequent documents which are conveyed and halted in succession.
However, this document conveying process has the drawback that three documents are conveyed simultaneously, so that the motor load increases, and a large current is necessary. In addition, because three documents are moving simultaneously, if a jam occurs, the next document overtakes the preceeding document from the rear so that there is the possibility of damage occurring to the documents. The margin for jam detection is small and jam handling is also complicated.
Also, in the above example, the gap between the rear end of the first document on the platen and the front end of the second document on the supply path is large, so the travel distance of the documents during document change increases, and accordingly, the time required for document change increases, which is a drawback.
In order to improve these drawbacks, the front end of the next document is conveyed as far as the contact nip position between the conveyor belt on the platen glass and the platen glass itself and made to standby at that point. However, the contact position of the conveyor belt is determined by the position of a document presser roller which applies pressure to the conveyor belt on the platen glass surface from the underside of the pressure belt, and the front end of the document does not come close to the preceding document.